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Given the disparity in numbers, it's possible that you've been blocked by some people. They would still count as having added you even if that means adding you to the list of people they don't want any interaction from.

Here's an excerpt from the block help from Google Plus that leans towards that theory:

They'll be removed from your extended circles even if you have mutual connections.

When you block someone, we won't proactively notify them. But since blocking someone limits the interactions that person can have with you, they may figure out that they've been blocked.

Circles in Google+ are asymmetric and private. You can't know what circle(s) you've been put in by other people and whether or not they're actively blocking you. But you will know if they've "added" you.

Where you end up is for them to know and you to scratch your head about.